11. Dual Fuel (Natural Gas Diesel) for Light-Duty Industrial Engines: A Numerical and Experimental Investigation

This paper reviews the main results of a numerical and experimental activity, carried out on an automotive four-cylinder, common rail, 2.8 L turbocharged diesel engine, Euro IV compliant. The purpose of the project is to convert this engine, with minor hardware modifications, in order to operate in compression ignition (CI) dual-fuel (DF) mode, using natural gas (NG) as the main source of energy. The diesel injector will keep the only function to ignite the homogeneous air–NG mixture within the cylinder, injecting just a small quantity of diesel fuel. In this way, soot emissions can be almost completely eliminated, and the after-treatment system can be strongly simplified (then, its cost reduced). Other fundamental advantages in the use of NG instead of diesel are the lower emission of CO₂ (provided that brake efficiency is not reduced when running on DF) and the lower concentration of nitrogen oxides (NO_x). This DF engine would be particularly suitable for light-duty industrial applications (power generators, small tractors, and off-road vehicles) and boats, where the installation of an additional fuel system is not limited by tight constraints. The experimental activity is supported by a comprehensive theoretical study, carried out through CFD simulation (both 1D and 3D). The numerical models are first calibrated for the standard combustion mode and then applied to get the guidelines for the development and calibration of the physical prototype. The most relevant experimental result is obtained at 3000 rpm, BMEP = 12 bar, where the DF engine can work with just a 20% of the diesel fuel required for standard operations. The following advantages are found: (1) complete elimination of soot; (2) 26% reduction of NO_x; (3) 25% reduction of CO₂; (4) slight improvement of brake efficiency. The only downside is the strong increase in HC and CO concentrations, which are about ten times higher. However, this issue can be addressed installing a cost-effective oxidation catalyst.